The prior art has only disclosed OCT (optical coherence tomography) systems and topography/OCT combined systems for measuring the biometric variables of the whole eye. Although Scheimpflug, PCI (partial coherence interference) and topography systems or combined instruments of same measure some of the aforementioned variables, these combinations cannot measure all parameters of the eye. In particular, it is not possible to measure the lens rear side and the retina, or the respective profile of these areas, since these systems, even in a combined system, are restricted to measuring the anterior chamber and the axial one-dimensional length of the eye.
Only OCT systems with anterior and posterior chamber measurements of the eye and topography/OCT systems, likewise with posterior and anterior chamber measurement, are able to measure the whole eye.
A further possible combination is a combination of topography/Scheimpflug system for the anterior chamber measurement and an OCT system for the posterior chamber measurement. However, since the OCT can also detect the anterior chamber, the gain from the Scheimpflug anterior chamber measurement is low compared to the additional costs.
Compared to the topography/OCT combined systems, pure OCT systems are disadvantageous in that measuring the topography of the cornea by means of conventional topometers (in particular Placido systems) is substantially more accurate than the measurements of the OCT systems which are influenced by movement artifacts. Although said OCT systems can reduce these movement artifacts by faster measurements or by measurements which are registered to the eye, this is only possible with significant outlay and not readily possible in a reliable enough manner.
By way of example, a combination of Placido topographs and time domain B-scan OCT is described as topography/OCT combined system in US2004/066489. In principle, this allows the whole eye to be measured biometrically. However, the described device exhibits some significant disadvantages, which reduce the reliability of the measurement values.
Although Placido topography has a very high resolution, it is less reproducible in terms of reconstructing the surface when compared to keratometer measurements. This is due, firstly, to the assumptions made during the reconstruction of the topography in order to achieve the high resolution and/or in the lacking telecentricity/insufficient focusability of many topography systems compared to keratometers, and so positioning errors of the measurement instrument in relation to the patient become relevant during the topography measurement.
Furthermore, Placido topographs do not allow so-called Skrew rays to be taken into account, which are always generated in the case of the Placido ring illumination when the cornea is curved not only in a central plane through the corneal vertex but also in a plane perpendicular thereto, i.e. if it has azimuthal curvature. As a result of not taking this into account, the corneal surface is not reproduced correctly. Thus, overall, a Placido topograph does not reproduce the radius or, in general, the front side of the cornea reliably enough as required for the IOL calculation.
Furthermore, time-domain OCT systems are too slow and competitively priced spectrometer-based systems do not have the axial resolution and/or have a too small axial scanning or detection depth such that the eye length does not occur with the resolution required for the IOL calculation or such that there are only partial depth measurements. However, whole-eye biometrics, i.e. establishing the areas of the whole eye optically relevant to the visual faculty of the eye in terms of their position and their profile in the eye, are, in principle, possible in both cases by separate measurement of the anterior and posterior chamber and subsequent synthesis of the data, but the registration of the images to one another is often unreliable due to lack of a suitable common reference variable in the segment images.
Therefore, the Placido time-domain OCT system does not allow all biometric data to be obtained in a sufficiently reliable and simple manner for the IOL calculation.
A combination of a simple keratometer and a B-scan OCT is described as a topography/OCT combined system in US20050203422. This system also allows important biometric variables of the biometrics of the eye to be determined. However, the described device also exhibits some significant disadvantages, which reduce the reliability of the measurement values or leaves open important points which are relevant to whole-eye biometrics:
The described keratometer merely allows the robust measurement of the radii on the front side of the cornea. A higher-order description of the corneal surface or a description with a higher resolution than that of the described keratometer is not possible. However, this is increasingly required for the calculation of intraocular lenses (abbreviated IOLs), in particular for toric IOLs.
Furthermore, this does not solve the problem of assigning the topography measured by the keratometer to the spatial data from the OCT data, nor does it ensure that the OCT measurements are taken particularly quickly in order to compensate for the eye movement during the measurement.